The present invention relates generally to centrifugal compressors and, more particularly, to a diffuser structure for centrifugal compressors.
One of the major problems arising in the use of centrifugal vapor compressors is that of maintaining flow stabilization when the compressor load varies over a wide range. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate. Accordingly, when there is a relatively low volumetric flow rate through such a compressor, the flow becomes unstable in the following manner. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there occurs a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are complete flow reversals in the diffuser passage, destroying the efficiency of the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates are desirable in most compressor applications, numerous modifications have been suggested to improve flow stability and machine efficiencies at low volumetric flow rates.
In U.S. Pat. No. 3,362,625, a vaneless diffuser is provided with flow restrictors which serve to regulate the flow within the diffuser in an effort to improve stability at low volumetric flow rates. In U.S. Pat. Nos. 2,996,996 and 4,378,194, there are described variable width vane diffusers wherein the diffuser vanes are securely affixed, as by bolting, to one of the diffuser walls. The vanes are adapted to pass through openings formed in the other wall, thus permitting the geometry of the diffuser to be changed in response to changing load conditions. Although a vaned diffuser is preferred over a vaneless diffuser because a vaned diffuser is more efficient at design incidence than a vaneless diffuser, the variable width vane diffusers presented a number of problems, particularly in regard to the manufacture, maintenance and operation of the machine. Such problems were overcome in the vaned diffuser shown in U.S. Pat. No. 5,807,071, wherein a pair of interconnected rings are provided to jointly define the flow passages which can be selectively varied by rotating one of the rings.
Another approach to a variable vaned diffuser is that shown in U.S. Pat. No. 5,683,223, wherein the individual vanes are selectively rotated in unison by way of a mechanism connected thereto to thereby accommodate the variable load conditions. Generally, such an arrangement is problematic in two respects. First, it is difficult to obtain the precise control that is needed in order to maintain uniformity in the positioning of the individual vanes. That is, for example, if it is desired that all vanes are closed, any inaccuracies in the positioning mechanism may well allow one or more of the vanes to be in a partially open position, thereby introducing inefficiencies that are undesirable. These nonuniformities are further complicated by the existence of various tolerances and the wear of components that are typical of such machines. Secondly, the substantial forces that are exerted on the leading edges of such variable position vanes, tend to cause vibration of the leading edges thereof to thereby affect dynamic stability. In order to control and or eliminate these vibrations it is necessary to provide a very strong, durable and stable vane positioning mechanism which is designed with these considerations in mind.
Although there are some prior art vaned diffusers which provide for the variable blade angle by rotation about a pivot point, the positioning of the pivot pin has not been optimized for best performance of the diffuser.
The object of the present invention is to provide, in a centrifugal compressor, a vaned diffuser, with the vanes being variably positioned and selectively controlled in order to effectively and accurately vary the pitch of the vanes in order to accommodate the variable load levels in the compressor.
In a preferred embodiment, a vane mounting means is provided with each vane having a pivot pin disposed near its leading edge and acting to position its vane, an actuation mechanism engaging each of the vanes near its trailing edge and operable to rotate the vane on the axis of its pivot pin, and a slot in each of the vanes to allow for relative movement between the vane and mounting means when they are relatively rotated. Such an arrangement provides for a positive and accurate positioning of the vanes so as to maintain a stable flow of gases therethrough.
In accordance with another aspect of the invention, the actuation mechanism includes a shaft and an associated eccentric cam surface which engages said vane, with the shaft being rotatable to cause the vane to rotate.
By another aspect of the invention, the pivot pin is integral with the vane.
By yet another aspect of the invention, the slot is located near the trailing edge of the vane and the cam surface is disposed in the slot.
In accordance with another aspect of the invention, the pivot pin is disposed in the slot.
In accordance with another aspect of the invention, the cam surface is round and is mounted in a round opening in the vane.
By another aspect of the invention, the actuation mechanism includes a ring which is interconnected to each of the vanes by way of actuation pins, and means for rotating the ring to move the vanes in substantial unison.
In accordance with another aspect of invention, the actuation pins are integral with the rotatable ring and are disposed in the openings formed in the vanes.
By yet another aspect of the invention, the vane openings are elongated to allow reciprocal movement between the actuation pins and the vanes.
By still another aspect of the invention, the actuation pins are integral with the vanes and are disposed in openings in the rotatable ring.
By yet another aspect of the invention, the location of the pivot pin has been optimized to reduce performance losses that would otherwise occur at the throats of the flow channels.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments considered in conjunction with the accompanying drawings.